Communication networks consist of interconnected nodes and can be subdivided into core networks and access networks, the latter providing access to user equipment, for example a wireless access for mobile user equipment to a radio access network. Core networks interconnect access networks and further networks, e. g. other core networks or the Internet. In the Universal Mobile Telecommunications System (UMTS) architecture, an access network can be controlled by a Radio Network Controller (RNC), which is connected to the core network and provides access to the core network, i.e. serves as access node. In the Global System for Mobile Communications (GSM) architecture, the access network can be controlled by a Base Station Controller (BSC). The 3G core network is controlled by one or more Mobile Switching Centres (MSCs). These MSCs also influence the decisions in RNC and BSC.
For the transmission on a connection, speech (or other media) is encoded (and subsequently decoded) according to one or more encoding/decoding schemes, also referred to herein as coding schemes and alternatively denoted “codecs”. Determination of an optimal codec or set of codecs may be done by means of Codec Negotiation. A coding scheme can transport speech either in a compressed or in a non-compressed mode. In many networks, different coding schemes can be used and different nodes can have different capabilities for handling the coding schemes. Speech transcoders perform the transcoding between different speech coding schemes, i.e. they decode the one scheme into speech (linear PCM or other representation) and then encode the speech by the other scheme. However a transcoder may also transcode speech directly between coding schemes without involving an intermediate representation of the speech. Hence, a transcoder is a device that performs a codec, i.e. it implements a particular coding scheme (in fact a transcoder can implement a number of coding schemes and employ them on per call basis as requested by call/session control applications). Tandem Free Operation (TFO) is a configuration of two transcoders with compatible coding schemes on the compressed voice sides at both ends of a connection, i.e. on the interface to the user equipment. In this case, the transcoding stages can be bypassed and the compressed voice coding is used end to end in the connection (see 3GPP TS 28.062).
Out of Band Transcoder Control (OoBTC) permits speech connections to be established end to end with a common coding scheme, i.e. ideally the same speech coding is used in the whole connection between the access networks. The advantage is that maintaining compressed voice saves core network bandwidth and optimizes speech quality, because transcoding stages, which in principle always introduce distortion, are avoided (see 3GPP TS 23.153).
An International Telecommunication Union (ITU) protocol called Bearer Independent Call Control (BICC) supports out of band signaling procedures, which allow a negotiation of the coding scheme between network nodes. In the ITU-Telecommunication Standardisation Sector (ITU-T) proposal BICC Q. 1901 (ITU, June 2000), coding scheme negotiation is performed from the originating control node in a connection to each subsequent node by including a list of allowed coding schemes in the Application Transport Parameter (APP) parameter in the Initial Address Message (IAM) for the set-up of the connection. Each node checks the list and if it does not support a particular coding type it removes it from the list. The adapted list is passed on with the IAM and any non-supported types are removed as long as the BICC signaling is supported. When the final node, either the terminating node or the last node supporting BICC, is reached, the coding scheme type is selected by the node. This selected coding scheme and the list of remaining, commonly supported codec schemes are returned to the originating node via all intermediate nodes.
In the BICC coding scheme negotiation procedures there are no rules for defining how many transcoder stages are allowed and whether an access network that supports out of band coding scheme negotiation can activate transcoders to keep Transcoder Free Operation (TrFO) between the access node and the rest of the network. The number of transcoding stages in a connection end to end can significantly affect the speech quality. More than three transcoding stages typically cause substantial speech impairment. The number of stages causing a substantial impairment depends on the coding algorithm/scheme and the speech impairment by further entities in the connection.
The coding scheme negotiation procedures may result in transcoders being activated to enable supplementary services or because the bearer technology in a node or network does not support compressed voice. For example, Asynchronous Transfer Mode (ATM) networks allow transmission of either compressed or non-compressed speech, while Synchronous Transfer Mode (STM) networks require non-compressed speech coding, which via bit stealing can include TFO with compressed speech (TFO is not really required in STM). Furthermore, it would be desirable that the negotiation should result in the controlled selection of the transcoders, which is with today's technologies not always the case.
In many cases, it is necessary to modify the coding scheme in a section of a connection. For example, a connection is often transferred between different access networks due to a handover. Also in this case a controlled selection of transcoders is often not achieved.
Therefore it is an object of the invention to provide an improved method for controlling the selection of a coding scheme to be employed in a communication network.